Chemical mechanical polishing (CMP) is a procedure used in the semiconductor industry to smooth the surfaces of wafers using a combination of chemical etching and abrasive mechanical forces. The wafer is placed on a rotating platen and held in place by a retaining ring as a polishing head is brought into contact with the wafer. Material is removed from the surface of the wafer, producing a planar surface. The removal rate may vary with pressure applied between the wafer and the polishing head.
In many cases, different regions of the wafer may polish at different rates (e.g., a wafer edge may polish faster than a wafer center). While polishing recipes can be developed and calibrated to compensate for different polishing rates in sets of wafers, inconsistent polishing may still occur from wafer to wafer due to surface irregularities despite prior calibrations.